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Microsoft Access Database Is The Bestseller Software.

February 21, 2010 By: lilybird Category: Software

MS Access is a relational database, which allows us to link tables via primary and foreign key. A primary key contains a unique value for every table record, such as an employee number. As for the foreign key, it exists in another table that is used to reference records in the main table. Both primary and foreign key must be defined by the same field size and data type. It might also be helpful to give the fields the same names, even though it is not necessary.

Primary key holds the vale only once, but foreign key can reference the value multiple times. So, a table with employee information will be referenced multiple times in another table, during the staff reviews every quarter. But this is only one type of relationship between tables. There are actually 3 types of Access table relationships.

The first type is one to one relationship, which occurs when you link two primary keys, so a record in one table has related record in another. The values of primary keys in both tables have to be alike for the relationship to work. You might consider splitting the employee data into several tables, because some information doesn’t need to be seen by other people besides certain staff members. And then, after splitting, you would link the two tables via this relationship.

The second type of relationship is one to many. This type is the most common and is often the first one to be explained. The training company, for example, would create a database with the records of all delegates stored in one table and courses records in another table. So, the table with courses will hold the foreign key for the records of the delegates, so they could be assigned for one or more classes. This kind of primary/foreign key relationship has unique value in the first table, which is referenced multiple times in the second table.

And the last type of table relationships is many to many. Basically, this is two sets of the previous type of relationship with a third junction table. As an example, we can use the same training company. And, again, we have the table with delegates’ records, the table with bookings and the courses table. The tables with courses and delegates will both be related to the bookings’ table using the one to many type of relationships. This will let each delegate to be assigned on multiple courses as well as allow each course have several delegates. In this case the table with bookings becomes the third junction table.

Linking your tables is an important skill to learn, when using the Access database, because it will add more functionality and give you more control over your database.

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